The present disclosure relates to a nitride semiconductor element, and more particularly to a nitride semiconductor element formed on a silicon substrate.
Gallium nitride (GaN) that is a nitride semiconductor has a high electron saturation velocity and a high breakdown electric field. Also, having high thermal conductivity, GaN is excellent in heat dissipation, and thus has a feature of being operable at high temperature. Moreover, highly concentrated two-dimensional electron gas (2DEG) is generated at the heterointerface between aluminum gallium nitride (AlGaN) and GaN due to the piezoelectric effect. Using 2DEG as a channel, large-current operation can be achieved, and thus implementation of low-loss, high-efficiency power devices represented by heterostructure field effect transistors (HFETs) is expected.
A nitride semiconductor layer having a good crystal structure can be formed easily if the substrate on which the nitride semiconductor layer is grown is a GaN substrate whose lattice constant matches with that of the nitride semiconductor layer. However, a GaN substrate is expensive, and therefore, it has been examined to form a nitride semiconductor layer on an inexpensive substrate such as a sapphire substrate and a silicon (Si) substrate. Since sapphire and Si are largely different in crystal lattice constant from nitride semiconductors, nitride semiconductor layers grown on such substrates are likely to have lattice defects. Lattice defects tend to cause leakage currents and current collapse. For this reason, attempts have been made to form an undoped GaN layer and then a superlattice layer, constructed of indium gallium nitride (InGaN) and AlGaN stacked on top of each other, on a sapphire substrate, thereby to reduce lattice defects of a nitride semiconductor layer formed on the superlattice layer (see Japanese Patent Publication No. 2001-274096, for example).
A sapphire substrate is less expensive than a GaN substrate, but is more expensive and smaller in substrate diameter than a Si substrate. Therefore, to further reduce the cost of power devices, it has been examined to form a nitride semiconductor element using a Si substrate that is inexpensive and easily available as a large-diameter substrate. Using a Si substrate, the difference in lattice constant from a nitride semiconductor layer is larger than using a sapphire substrate. In addition, the difference in thermal expansion coefficient between a Si substrate and a nitride semiconductor layer is very large, resulting in that cracks tend to be generated in the nitride semiconductor layer grown on the Si substrate. To reduce occurrence of cracks, therefore, it has been examined to form a superlattice layer, constructed of a GaN layer and an aluminum nitride (AlN) layer stacked on top of each other, between the Si substrate and the operation layer (see Shinichi IWAKAMI, Masataka YANAGIHARA, Osamu MACHIDA, Emiko CHINO, Nobuo KANEKO, Hirokazu GOTO, and Kohji OHTSUKA, “AlGaN/GaN Heterostructure Field-Effect Transistors (HFETs) on Si Substrate for Large-Current Operation,” Jpn. J. Appl. Phys., 2004, vol. 43, p. L831).